Imaging of materials

ABSTRACT

An apparatus and method for obtaining an image of an object. The apparatus comprises a radiation source and a series of two to five linear detectors spaced therefrom to define a scanning zone; means to cause an object to move relative to and through the scanning zone; a direct image generation apparatus to generate an image from the output of a linear detector; an intermediate image generation apparatus configured to generate at least one intermediate image from an adjacent pair of linear detectors, by processing the output of the pair of detectors and generating an image representative of an output intermediate between the two said detector outputs; the image apparatus adapted such that at least five and preferably at least six images are generated in total; an image display adapted successively to display such images and thus display the monocular movement parallax between the images.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for detecting, imagingand in a preferred embodiment, characterising material inthree-dimensional space.

The invention in particular relates to an apparatus and method makinguse of high energy radiation such as X-rays or gamma-rays to scanobjects where it is desirable to gain information about their internalcontents and/ or composition. The invention in particular relates to anapparatus and method operating on the line-scan principle, in whichthree dimensional objects are caused to move through a scanning zone andimaging information collected. These principles are widely employed forexample in the security industry, to scan objects where it is desirableto gain information about their internal contents, but might also beemployed in other areas, for example, without limitation, medicalimaging, imaging for quality control purposes or the purposes ofdetermining the integrity of the structure, or the like.

BACKGROUND

Imaging apparatus which employs the line-scan principle is well known.Typically, such apparatus will consist of a high energy radiation sourcesuch as an X-ray source, and for the purpose of exemplification hereinfurther discussion will describe X-ray systems in particular. The beamof the source may be collimated into a curtain, usually referred to as a“curtain beam”, and is then detected by a linear array detector forexample comprising a linear photodiode array. Image information isobtained by having the object of interest move linearly for example atright angles with respect to the beam and storing successive scans ofX-ray transmission information derived from the linear array from whicha complete image frame can be compiled.

If the object being scanned is heterogeneously transmissive of x-rayradiation, and for example consists of or contains multiple smallerobjects and/or components of dissimilar materials, it can be possible tobuild up an image of the object, and in a particular case of thecontents or components. The image may then be displayed on a viewingscreen. This image can be useful for example in relation to the possibleapplications outlined above. In particular, it can be useful indetermining the contents of a container or the internal structure of anobject or body.

Even so, the image generated by such an X-ray apparatus is limited. Atbest it constitutes a two dimensional shadowgraph of the object beingimaged. This can make it difficult to interpret.

European Patent No. 610084 describes a method of creating a “2.5D” solidmodel picture for viewing. A stereoscopic pair of X-ray images isobtained using two diverging curtain beams derived from an X-ray source.These are separated into conjugate slices and the 2.5D image built upfrom the resulting slice information.

The resultant image is not strictly a three dimensional image (althoughit is often so referred to) since it is presented on a two dimensionalscreen rather than by means of full stereoscopic apparatus. Such a 2.5Drepresentation in fact contains psychological cues to depth such aslinear perspective, interposition, shading and shadowing rather than thefull physiological depth cue known as binocular parallax or stereoscopywhich is required for a full three dimensional image.

The method of EP610084 still provides a user with a final image whichcan be rotated and looked at from different directions and which cangive considerable information as to the relative positioning ofdifferent objects or compounds. However it does not give information asto the nature of the items which have been located.

UK Patent Nos. 2329817 and 2360685 are examples of methods and systemswhich can be used to produce full stereoscopic image pairs. They deriveultimately from principles set out in EP0261984. In particular they aresubject to the condition set out at column 4 lines 31 to 48 thereinwhich imposes considerable constraints on detector and source beamgeometry. Although stereoscopic imaging can be a relatively powerfultechnique, exploiting full physiological cues in relation to depthinformation, and thus offering the potential for a user of the X-rayapparatus to identify objects or components much more readily andclearly, it can be complex in practical operation. To exploit thestereoscopic effect, it is necessary for the observer to receivedifferent images at each eye simultaneously. This will necessitate theuse of special apparatus. Moreover, a full stereoscopic techniquerequires precise control of the image collection process having regardto the conditions identified above. If the stereoscopic pair is to beeffective, the respective images must be collected with a parallax thatclosely approximates to that which would be tolerated by the observer'seyes. For these reasons, full stereoscopic imaging has not gained wideacceptance for scanning machines of this type.

Not only do conventional non-stereoscopic apparatus and methods tend togive limited information in a third dimension, but also the images theyproduce also give limited information about the material content. Inessence, at its simplest, all that is being measured is X-raytransmissivity.

In most practical systems even this is measured indirectly. At itssimplest, a typical linear array detector comprises in combination ascintillator material responsive to transmitted X-rays, which is thencaused to emit lower frequency radiation, and for example light in oraround the visible region, in combination with a semiconductor detectorsuch as a silicon or gallium arsenide based detector which is responsiveto this lower frequency radiation. The detector merely collectsamplitude information, and does not discriminate spectroscopically.

However, it is known that spectroscopic information from the transmittedX-rays could be used to give additional information about the materialcontent of the objects or components being scanned. This has led todevelopment of dual band detectors, which are capable of separatelyidentifying low and high energy bands from the full spectrum of X-rayemissions. Such a dual energy sensor typically comprises a sandwich pairof semiconductor photodiode arrays or the like, in conjunction with ascintillator configuration that is configured such that the respectivedetectors detect transmission of low-energy and high- energy X-rays. Itis known that the X-ray absorption properties of any material can varyspectroscopically, and that the amount by which the absorptionproperties vary depends in particular on atomic number. This isexploited by the dual energy detector to differentiate generally betweenobjects having lower and higher atomic number elements predominating.

When exploited as part of a security or material identification system,a very crude approximation can be made that organic materials tend to bein the former category and most inorganic materials in the lattercategory. However, even such a system confers only limited informationabout composition. The organic/inorganic division is crude andapproximate, can readily be confused by objects which are superimposedin the X-ray path, and will give no information concerning thecrystalline or polycrystalline nature of an object.

UK Patent Nos. 2329817 and 2360685 incorporate dual-energy transmissiondetectors. Even so, the compositional information given by thearrangement is still limited, for example in that the low/higher energyduality effect can give only a crude approximation of anorganic/inorganic split and cannot itself distinguish polycrystallinematerials.

For this reason, the references include additional scatter detectors.X-rays are scattered by the materials they pass through and thesescattered signals can contain information that may be used to identifythe scattering materials. There is great applicability for thesedetectors as many of the materials that raise security issues, such asexplosives, drugs and semiconductor materials, have a polycrystallinestructure and therefore produce good scatter signals. This technique ofidentifying materials from the scattered signal though possible is notcurrently commercially used as the extra scatter detectors introducegreater complexity in the system and the scattered beams are weak and sothroughput is limited.

The line-scan X-ray technique is widely used in security applicationswhere the detection and differentiation of objects of complex and variedshape and composition is an important feature. A better resolution ofthe exact shape and location of such objects in three-dimensional spacewould be a considerable improvement on present techniques, especially ifcomposition could also better be characterised.

SUMMARY OF THE INVENTION

It is an object of the present invention to mitigate some or all of theabove disadvantages of prior art line scanning systems.

It is a particular object of the present invention to provide a methodand apparatus for line scanning of objects, and especially of containersof multiple objects or objects comprising multiple components, whichprovides additional information about their shape and/or location inthree-dimensional space.

It is a particular object of the invention to provide a method andapparatus which generates an image providing information about the shapeand/or location of objects in three dimensional space which does notrequire special viewing apparatus but can be presented effectively on atwo dimensional viewing screen.

Therefore, according to one aspect of the invention there is provided anapparatus for generating and displaying an image of an objectcomprising:

a high energy radiation source such as an X-ray or gamma-ray source anda series of at least two and no more than five linear radiationdetectors such as, as applicable, X-ray or gamma-ray detectors spacedtherefrom to define a scanning zone therebetween;

means to cause an object to move relative to and through the scanningzone in use;

a direct image generation apparatus configured to generate an image fromthe output of at least one and preferably each linear detector;

an intermediate image generation apparatus configured to generate atleast one intermediate image from at least one and preferably eachadjacent pair of linear detectors, by processing the output of the oreach said pair of detectors and generating an image representative of anoutput intermediate between the two said detector outputs;

the image generation apparatus being adapted such that at least five andpreferably at least six images are generated in total;

an image display adapted successively to display such images and thusdisplay the monocular movement parallax between the images.

By analogy, in accordance with a further aspect of the invention thereis provided a method of obtaining an image of an object comprising thesteps of:

providing a high energy radiation source such as an X-ray or gamma-raysource and a series of at least two and no more than five linearradiation detectors such as, as applicable, X-ray or gamma-ray detectorsspaced therefrom to define a scanning zone therebetween;

causing an object to move relative to and through the scanning zone;generating a direct image from the output of at least one and preferablyeach linear detector;

generating at least one intermediate image from at least one andpreferably each adjacent pair of linear detectors, by processing theoutput of the or each said pair of detectors and generating an imagerepresentative of an output intermediate between the two said detectoroutputs;

such that at least five and preferably six direct and intermediateimages are generated in total;

displaying such images successively and thus displaying the monocularmovement parallax between the images.

Thus, in accordance with the method and apparatus of the first andsecond aspects of the invention successive images are generated asrelative movement is effected between an object and a source/detectorarrangement. Suitable means are provided to effect relative movement ofobject and scanning zone, for example comprising an object handler. Itmay often be convenient for an object to be moved, for example by asuitable object handler or conveyor, through a static scanning zone, butit will be appreciated that it is also valid to have the object remainstatic and to translate the detector and source to create the relativemovement.

At least five and preferably at least six images (direct andintermediate) are generated from at least one spaced pair of linearX-ray or other radiation detectors successively reflecting movement ofan object relative to a scanning zone. However, although a plurality ofimages is generated, the images are not simply collected and displayedas paired images with the intention of providing a stereoscopic effect.Rather, the invention exploits the monocular movement parallax betweenthe successive images.

This parallax can give some effective physiological cues in relation toobjects located in three-dimensional space, in particular if asuccessive multiple series of images is used, although the effect willbe present for any pair of images. However, it has a number ofadvantages of simplicity over a full stereoscopic system.

In particular, the images are displayed successively as individualtwo-dimensional images, and require no special apparatus. The systemexploits a monocular parallax. It is not necessary simultaneously togenerate different images for each eye of a user as would be the casefor stereoscopic viewing. A binocular or other complex stereoscopicviewing apparatus is not required. A simple monocular display issufficient. Moreover, the relatively strict control of image productionwhich is necessary to exploit binocular parallax effectively does notapply. For example for an effective binocular parallax pair of images,the images need to be generated in such a way that the parallax betweenthem closely matches that which can be tolerated by a human observer. Bycontrast, an effective monocular parallax illusion of movement in threedimensions can be produced by a much greater range of successive images.It is not necessary to comply with the narrow conditions for beamgeometry that apply to stereoscopic imaging in the prior art as set outnumerically as noted above in EP0261984. The technique is thuspotentially much more robust in practice.

The method and system in accordance with the first and second aspects ofthe invention are thus potentially simpler in many practicalcircumstances than a binocular method and apparatus, and, significantly,the invention tends to lend itself more closely to application withlimited modification to existing apparatus and techniques. For examplein the security field where X-ray line scanning using linear arrays ofX-ray detectors is widely used, images produced in accordance with themethod of the invention can be displayed on a two dimensional videodisplay screen and information can be obtained from such images by auser seated at such a screen in an essentially conventional manner.

The monocular parallax effect is present in theory in just a pair ofimages, and visual cues in a third dimension which enable a user toresolve objects, and in particular individual contents and/or componentsthereof, being scanned can be obtained from such a simple pair ofimages. However, the effect is enhanced, and in particular a moreeffective impression of movement in three dimensions can be obtained, ifa series of at least five and preferably at least six successive imagesis generated and displayed. In accordance with the invention, at leastfive and preferably at least six images are generated.

There are two ways in which additional images can be generated. First,there may be provided a laterally spaced series of more than two linearX-ray or other radiation detectors, for example at least threedetectors. The output of each such detector may then be used singly togenerate an image representative of data collected at the detector.However, a large number of linear detectors is not necessarily desirableand may require more complex apparatus.

In accordance with the invention therefore, further images are generatedby interpolation of pairs of images collected from adjacent lineardetectors. Thus, in accordance with the invention, the method comprisesgenerating at least one intermediate image from at least one of andpreferably a plurality of and for example each adjacent pair of lineardetectors, by processing the output of the or each said pair ofdetectors and generating an image representative of an outputintermediate between the two said detected outputs. The apparatus of theinvention includes an intermediate image generator comprising a meanshaving this functionality.

Sufficient intermediate images are generated to create at least five andpreferably at least six images in total (that is, when direct imagescollected from the direct detected output of each detector andintermediate images generated by interpolation are summed) from no morethan five detectors, and such that in any event more images aregenerated than there are detectors. Where only two or three detectorsare used additional images are created by synthesising more than onevirtually spaced intermediate images between sufficient pair ofdetectors to result in the required number of images. The intermediateimage generator is then configured to generate two or more intermediateimages from at least one adjacent pair which show a gradual transitionbetween the images produced by each adjacent pair by processing theoutput of the or each said pair of detectors and generating imagerepresentatives of a transition between the two said detected outputs.

Thus an apparatus of the invention generates at least five andpreferably at least six images, which is viewed as an effective minimumfor optimum resolution of the parallax effect by a human viewer, but inall cases with fewer than six linear detector arrays and preferably inall cases producing more images than there are detector arrays.Sufficient visual information is generated from a simplified apparatus.

Where a single intermediate image is generated between an adjacent pairof detected images, the intermediate image preferably extrapolates imagepoints on the intermediate image to a mid-point between equivalent imagepoints on the two detected images. Where a plurality of intermediateimages is provided between a pair of detected images these arepreferably based on a similar, evenly spaced extrapolation. Furtherpreferably, the relative spacing of detectors and the configuration ofintermediate image generation is such that all images are generated withapproximately the same relative movement spacing. For example detectorsare evenly spaced and the same number of intermediate images isgenerated between each pair of a plural array.

In accordance with the foregoing, both the direct and the indirectapproach are used to generate a series of images of an object in ascanning zone in what amounts, in effect, to different positions. In thefirst approach, a plurality of spaced detectors produce a correspondingplurality of direct images. In the second approach, actual data fromadjacent linear detectors is used to generate an indirect imageintermediate to the direct images, and thus representative of anintermediate object position.

Both methods are used to generate a successive series of images fromwhich, by means of the monocular movement parallax between each pair ofsuccessive images, information about a third dimension, and inparticular an impression of movement through a third dimension, can beobtained. A relatively small number of images is required to produce aneffective animation-like movement in three dimensions by a userobserving a simple two-dimensional screen.

In accordance with the invention at least five and preferably at leastsix images are generated but, as a result of use of the interpolationmethod to generate at least one indirect image from the output of eachpair of detectors five or fewer detectors are required.

Where a plural series of laterally spaced linear detectors is provided,the space in between them is preferably generally constant, for examplein that each pair of adjacent linear detectors in the series issubstantially equally laterally spaced and/or at a substantially equalangular spacing relative to the X-ray or other radiation source.

The linear X-ray or other radiation detectors making up the laterallyspaced series are preferably generally parallel. They may be disposed ina common plane or along a common arc. Each linear detector may be of anysuitable configuration, for example of a conventional simple or foldedconfiguration.

Preferably the linear detectors making up the laterally spaced seriesare arranged such that the distance between them changes in order tomaintain a constant angular separation between each array. This is ofcritical importance for L-shaped detectors which are commonly found inmany scanning industries. This innovation is also of critical importancewhen the source of the radiation is not positioned at the central pointof each array.

Preferably the apparatus comprises alignment means 4 to allow forprecise alignment of each linear detector as above described.

Preferably the apparatus comprises means to deliver plural beams,preferably from a single radiation source. For example a beam isdirected at each linear detector. Preferably the means to deliver pluralbeams allows a user to select any of two to five beams as the case maybe. Conveniently the apparatus comprises a collimator adapted to allow auser to select two to five beams from a single radiation source.

Conveniently, the successive images are displayed under observercontrol, for example in that control means are provided to enable a userto display successive images under control, and for example at a chosenspeed and/or in a forward and reverse order on the image display means,so as to create an effective manipulation of third dimensional cues fromthe image series. For example this allows a user to display successiveimages at variable speeds in order to enable a smooth transition for thehuman observer between each image and/or to run the images in a forwardand reverse order. In accordance with the method of the invention thesuccessive images are thus displayed at custom definable refresh ratesand directions under observer control to facilitate interpretation

The display means is conveniently a simple two dimensional displayscreen, for example a conventional video display screen (which term isintended to encompass any direct display or projection system exploitingany cathode ray tube, plasma display, liquid crystal display, liquidcrystal on silicon display, light emitting diode display or liketechnology). It is a particular advantage that the method can beenvisaged for use with, and the apparatus for the invention incorporatedinto, the standard display screens of comparable existing systems forexample in the security and medical imaging fields.

The X-ray or other radiation source is in particular preferably acurtain beam source as will be familiar from conventional line scanapparatus. The source may comprise a single primary source adapted togenerate a series of beams such as curtain beams aligned to be incidentupon each linear detector in the laterally spaced serial array at asuitable angular separation, from example by a suitable beam splittingapparatus. Alternatively, multiple sources may be provided eachgenerating a beam such as a curtain beam incident upon a linear detectorin the serial array. The source may comprise a source combining both ofthe foregoing principles.

Each linear detector in the laterally spaced series is a detector forthe detection of transmitted X-rays or other radiation. For example,each detector is adapted to generate an electrical impulse in responseto transmitted X-rays or other radiation, thus enabling data to becollected from which an image can be constructed as an object is causedto move through the scanning zone. Each detector conveniently exhibits aphotoelectric response and may for example comprise a linear array ofphotodiode cells. The detector/cell may comprise a material directly andinherently capable of generating such an electrical response to incidentX-rays or other radiation, or may comprise a combination of materials togenerate such an electrical response indirectly, for example comprisinga scintillator layer generating lower frequency electromagneticradiation in response to incident X-rays or other radiation, and amaterial generating an electrical signal in response to such lowerfrequency incident electromagnetic radiation.

A conventional simple linear detector is of this latter type. A sandwichlayer comprising a scintillator material, which generates photonsgenerally in or in the vicinity of the visible region from incidentX-rays, and a semiconductor layer, typically based on silicon or galliumarsenide, which generates an electrical signal via a photoelectriceffect from the incident visible light, is used. Thus, the detectormaterials together generate an electrical impulse representative oftransmitted X-rays incident upon the sandwich, and from which an imagecan be built as above described.

Successive images, even using a simple detector of this type, can beused in accordance with the method or apparatus of the invention togenerate potentially significant third-dimensional cues which cansignificantly enhance the user resolvability of the resultant images,and in particular enhance the information which can be obtained inrelation to multiple objects, or to an object having multiple contentsor components. However, no compositional information is given.

In an alternative embodiment at least some of the linear detectorscomprise dual energy detectors. This term is used herein to refer todetectors which can differentiate between low- and high-energy X-rays.The dual energy X-ray transmission part of the composite detector couldbe based on scintillator materials and photodiodes for example bycomprising a sandwich structure of scintillators and semiconductormaterials as above described, or could be based on scintillators inconjunction with optical fibres. In each case the emitted X-ray spectrumis filtered into high- and low-energy regions, and thus provides somecompositional information based on atomic number (which to a crudeapproximation can differentiate to some extent between organic andinorganic materials).

If each X-ray image obtained is detected by a dual energy sensor as anobject being examined is moved through the scanning zone, the dualenergy sensor provides information about the nature of the object. Thiscan further assist, in combination with the three dimensional cues whichare provided by the monocular movement parallax between successiveimages, to assist in resolution of multiple objects or components,particularly those which are superimposed in an individualtwo-dimensional image.

However, it must be emphasised that conventionalscintillator-semiconductor detectors do not give any real spectroscopicinformation about the spectrum of transmitted X-rays. These simpledetectors merely detect the presence or otherwise of transmitted X-rays.Even the dual energy detectors ultimately operate on the same principle,although they detect the presence or otherwise of X-rays within twodistinct bands of the spectrum.

In accordance with a particularly preferred embodiment of the apparatusinvention therefore, at least some of the linear detectors in the seriescomprise detectors that can generate spectroscopic information about thetransmitted X-rays or other radiation. That is, the detector exhibits aspectroscopically variable response across at least a substantial partof the X-ray or other radiation source spectrum allowing spectroscopicinformation to be retrieved.

Proper resolution of spectroscopic information confers two advantages.It offers the potential directly to characterise the composition ofdifferent components or objects, or parts of the image, and bydistinguishing between objects, components or parts of differentcomposition, for example by representing them differently (such as indifferent colours) in the resultant image, it assists in resolution ofdifferent objects, components or parts of the image.

Correspondingly in accordance with a preferred embodiment of the method,collected transmission data is resolved spectroscopically.

In accordance with this preferred embodiment, spectroscopic resolutionof transmitted X-rays or other radiation obtained from the output ofeach such linear detector is represented in the generated image. Forexample, spectroscopic differentiation in the collected data isrepresented in the image as differentiated colour, shading or marking.For example, a functional mapping is created between the spectroscopicinformation collected by the detector and the visual spectrum, and acoloured image display derived according to this functionalrelationship. Alternatively, a banded mapping is used in that thespectrum is divided into a plurality of bands, for example between fourand eight bands, and different colours are used to represent each suchband in the displayed image. The apparatus conveniently includessuitable image processing means to effect this mapping.

It is necessary that the detector system is enabled to detect radiationin a manner which is spectroscopically resolvable by the data processingapparatus. Preferably, a linear detector system in accordance with theinvention, or some or all discrete detector elements making up amulti-element detector system in accordance with the invention, may beinherently adapted to produce spectroscopic resolution in that itexhibits a direct spectroscopic response. In particular a system orelement is fabricated from a material selected to exhibit inherently asa direct material property a direct variable electrical and for examplephotoelectric response to different parts of the source spectrum. Forexample, the detector system or element comprises a wide direct bandgapsemiconductor material. For example, the detector system or elementcomprises a semiconductor material or materials preferably formed as abulk crystal, and for example as a bulk single crystal (where bulkcrystal in this context indicates a thickness of at least 500 μm, andpreferably of at least 1 mm). The materials making up the semiconductorare preferably selected from cadmium telluride, cadmium zinc telluride(CZT), cadmium manganese telluride (CMT), germanium, lanthanum bromide,thorium bromide. Group II-VI semiconductors, and especially thoselisted, are particularly preferred in this regard. The materials makingup the semiconductor are preferably selected from cadmium telluride,cadmium zinc telluride (CZT), cadmium manganese telluride (CMT) andalloys thereof, and for example comprise crystalline Cd_(1−(a+b))Mn_(a)Zn_(b)Te where a and/or b may be zero.

Combination of these and any other such materials may be consideredwhich give spectroscopic detection rather than merely detectingamplitude of transmitted radiation.

The use of linear array detectors which confer full spectroscopicinformation is particularly preferred as, in addition to the inherentadvantages of giving compositional information from the spectroscopy,such an ability to identify materials will operate co-operatively withthe ability to obtain third dimensional cues via monocular movementparallax to assist further in the resolution of multiple objects,components or image elements in the third dimension.

The basic geometry comprising at least two linear array detectors inserial array, and preferably a plurality of the same, to allow thegeneration of multiple images and to view these as an animated sequenceis fundamental to the invention in providing an enhanced ability tointerpret the shape and form of the object.

However, the greatly improved materials identification capabilityconferred by using spectroscopic linear detectors enhances both theability inherently to identify composition of objects and theresolvability of different objects in an image via a number of stepswhich could be additive and reinforcing.

In particular for example spectroscopic detectors can be operated in anenergy selective manner, giving rise to the ability to present an imageresolved into a significantly increased number of X-ray or otherradiation energy bands compared with the two that are available fromstandard prior art dual energy detectors. This information can be usedto provide a much higher degree of material identification than iscurrently possible and also, by separately identifying objects ofdifferent compositions within a composite image, to improveresolvability of such objects.

As an additional effect, the way in which the geometry is arranged withmultiple linear detectors arrayed in series across the scanning zonemeans that each of the ray paths seen by an individual detector isdifferent. Again spectroscopic detectors will be able to gain much moreinformation via these different paths. Some similarity can be seen tothe multiple ray path technique used in standard CT technology.

By combining the effects of both the above two aspects of spectroscopicresolution using appropriate algorithms a much more preciseinterpretation of the type of material being scanned can be achieved.

In their primary mode of operation a laterally spaced series of lineararray detectors is configured to receive multiple beams from the sourcewhich may for example be from a single X-ray or other radiation source.In a particular embodiment of this mode of operation, one or moresources may be used to generate a series of incident beams and forexample curtain beams at relative angles appropriate to the distributionof the separate linear detectors. In this mode of operation, the methodand apparatus of the invention may exploit the foregoing advantages.

The geometry also admits an alternative mode of operation. In accordancewith this alternative mode of operation a single primary curtain beam isgenerated, for example directed generally towards a mid-point of thelaterally spaced series of linear array detectors, and in particular inthe direction of a linear detector at or about the mid-point of theseries. The apparatus of the invention preferably includes a suitablecollimator to collimate the output of the X-ray source to produce such aprimary beam.

With an object in place, this single primary beam is directed towards acentral linear detector within the series of detectors. In thesecircumstances, this detector detects transmitted X-rays, but the otherdetectors could only be detecting diffracted secondary beams. Subject toappropriate calibration, signals that appeared on the secondarydetectors would give additional information concerning scattered X-rays.It is known that scattering is characteristic of polycrystallinematerials, which structure is relevant in relation to many of theexplosive and like materials that a security detector apparatus might bedirected at identifying. Accordingly, in this mode of operation, asignificant enhancement of the detectability of such items is offered.This is achieved without the need for secondary scattering detectors butrather by suitable processing of data from the secondary mode ofoperation.

In a particular preferred embodiment, both modes of operation may beapplied sequentially or closely successively or effectivelysimultaneously to collect maximum information. That is, images may bebuilt up using a single primary beam directed through the full scanningzone created by the linear detectors in the series, and subsequently orclosely successively or effectively simultaneously a single collimatedprimary beam may be directed at an individual linear detector, such as agenerally centrally located linear detector in the series, and scatterinformation collected from the other detectors.

It is a particular advantage of the preferred embodiment, wheredetectors are used which are capable of resolving informationspectroscopically, that the data collected by these two methods can beresolved more readily to give substantial enhancement to thecompositional information obtainable by operation of the system.

For operation of the method of the invention it is necessary to causerelative movement between an object to be scanned and the apparatus soas to cause the object to be scanned to move relative to and through thescanning zone. In practice for most practical purposes an object ismoved relative to a stationary-scanning zone. For example the apparatusof the invention includes object conveyance means to convey an object tobe scanned through a scanning zone. The object conveyance means may forexample comprise of planar conveyor, which preferable conveys an objectin a plane parallel to a plane in which the series of linear arraydetectors is disposed. For example, the conveyor is an endless beltconveyor or the like. However it is also valid to have the object remainstatic and to translate the detector and source to create the relativemovement.

The source must produce a distribution of energies. Preferably thesource is an X-ray source. Tungsten is the most appropriate target, butothers could be used.

One or more of these features of the invention can be combined so as toprovide enhanced information for an operator and the specific embodimentdiscussed below shows such combinations.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example, with reference tothe accompanying drawings in which:

FIG. 1 is a side view of a representation of an embodiment of theinvention;

FIG. 2 illustrates the effect created by successive images generated bymeans of the embodiment of FIG. 1;

FIG. 3 is a side view of the embodiment of the invention in analternative mode of operation; and

FIG. 4 is a schematic representation of an image processing and displaysystem.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to FIG. 1, a suitable X-ray source 1 is used to direct X-raysvia a scanning zone in the direction of three linear array detectors 3 ato 3 c.

In the embodiment, the linear array detectors 3 a to 3 c comprisematerial capable of spectroscopic resolution of incident X-rays, and inthe specific example comprise cadmium telluride although the skilledperson will appreciate that other material selections may beappropriate. To exploit this spectral resolution, the X-ray source emitsX-ray across a broad energy spectrum. In the example a tungsten sourceis used, although the skilled person would appreciate that othermaterials might be appropriate.

Incident ray paths 5 a to 5 c are shown through the scanning zonebetween the X-ray source 1 and, respectively, the detectors 3 a to 3 c.

An endless belt conveyor 7 causes an object to be scanned 9 to move in adirection d so as to intercept the ray paths 5 a to 5 c in the scanningzone. The envisaged application of this embodiment of the invention isas a security scanner, and object 9 can be considered typically to be acontainer that is expected to contain a variety of distinct objectswhich it would be useful and desirable to characterise compositionallyand to view effectively in a third dimension. However, the skilledperson would readily appreciate that the same principles can be appliedfor example to the scanning of objects for internal examinationpurposes, to medical scanning, and to similar applications.

Images are generated by building up transmitted information from each ofthe three detectors 3 a to 3 c. The image processing system isrepresented schematically in FIG. 4.

Referring to FIG. 4, as an object passes through the scanning zone datais collected from the three detector arrays 3 c to 3 c and transmittedto a data collection and processing unit 21 which assembles and collatesthe data and produces therefore individual images characteristic of thecollected data from each detector. These are passed to an image storageregister 22.

The data collection and processing unit 21 includes a direct imagemodule and an intermediate image module (not separately identified) forexample in the form of suitable processing software. The direct imagemodule generates an image dataset representing the incident informationdataset at each of the three detector arrays 3 c to 3 c. Theintermediate image module generates virtual image datsasets based onnumerical processing of the incident information datasets of eachadjacent pair of detector arrays 3 c to 3 c. In the illustratedembodiment the intermediate image generator is configured to generatetwo or more intermediate images from each adjacent pair which show agradual transition between the direct images produced by each adjacentpair by processing the output of the or each said pair of detectors andgenerating image representatives of a transition between the two saiddetected outputs.

The key to the interpretation of the images in the manner envisaged bythe invention lies in the way that the images are displayed. A simpledisplay, 25, comprising in this example a two-dimensional video displayscreen is provided. Images are passed from the image register 22 to bedisplayed thereon successively and sequentially. The result of thissuccessive sequential display is that an observer viewing the screen 25is able to glean information from the successive images, and inparticular third-dimensional cues, by means of monocular movementparallax between them.

A particular feature of this is illustrated by FIG. 2. As an object 9passes through incident ray paths 5 a to 5 c (see FIG. 2 a) three imagesare generated in which the object is oriented differently relative tothe X-ray source 1. Successive display of these images will cause theobject to appear to rotate about as is illustrated in FIG. 2 b.

This ability in effect to get a view of the object which is in effectrotatable in a third dimension can be seen to be analogous to the visualbenefits that are seen in a volume rendered CT image. The ability toview objects in pseudo 3D assists the human observer in making correctassumptions about the object in view. The image generated in accordancewith the present invention offers these benefits as a result of themultiple ray paths provided by the apparatus, but with a less complexgeometry than typical CT machines, and for example may be easilyimplemented on a simple linear conveyor such as is used in securityscanning systems.

FIG. 2 illustrates directly collected images only for clarity. As theembodiment has three detectors, three directly collected images of theobject in different positions can be generated. However, in addition todirectly obtained images, the image processor 21 is adapted to generateintermediate images by extrapolation of the directly collected data.

In one possible embodiment a single intermediate image is generated foreach collective pair. Thus the data register 22 stores five images whichcan be displayed sequentially. In a second possible embodiment twovirtually spaced intermediate images are generated for each collectivepair. Thus the data register 22 stores seven images which can bedisplayed sequentially. Other array and intermediate image protocols maybe applied.

Thus, the data collection and processing unit 21 in the illustratedembodiment generates at least seven images from only three detectors.Thus the data register 22 stores at least seven images which can bedisplayed sequentially. Other array and intermediate image protocols maybe applied, subject to the underlying principle that six or more imagesrepresenting real or virtual position shifts and thus with relativemovement parallax are generated by five or fewer detectors.

The sequential display of images on the screen 25 is under user controlby the control means 27. This allows a user to generate animatedsequences from the succession of images stored in the data register 22,and effectively to manipulate the collection of images to gainsignificant third dimensional cues from the collected data. Optionallyto further enhance this the control means 27 includes a means to controla belt drive 29 which acts to operate the conveyor belt 7 allowing theobject itself to be passed and re-passed through the scanning zone incases of uncertainty.

In the illustrated embodiment, detectors are used which are capable ofresolving the transmitted X-rays spectroscopically. This resolvedspectroscopic information forms part of the data that is subsequentlyprocessed to provide materials identification of the objects in theimage.

The particular advantages of the present invention over more complexstereoscopic systems are well illustrated by FIGS. 1 and 4. Noparticular stereoscopic viewing apparatus is required, but merely atwo-dimensional screen 25. The geometry of the system lends itselfsimply to a belt conveyance apparatus, and thus the system of thepresent invention requires little fundamental change in the basicapparatus features or method of use when compared with a conventionalX-ray scanner. However, it offers a significant ability to enhance bothobject resolution in three dimensions and material characterisation, inparticular thanks to the effect illustrated in FIG. 2.

FIG. 3 represents an alternative mode of operation for the scanner, andis illustrated with an object 9 in position in a scanning zone.

The X-ray source 1 has its output collimated into a single curtain beam13 by a collimator 11. The single path beam is directed at the middledetector 3 b. The middle detector 3 b receives transmitted X-ray via theray path 15 b. The other detectors 3 a and 3 c receive scattered X-raysvia the respective ray paths 15 a and 15 c. Thus, the apparatus of theinvention can readily be adapted via this mode of operation to detectboth transmitted and scattered X-ray simultaneously, and to make use ofthe information derivable therefrom to characterise polycrystallinematerials. Separate scatter detectors are not required. The resolutionof transmitted and scattered X-rays is assisted in the embodiment by thespectroscopic resolution conferred by the cadmium telluride detectors.

Data from the mode of operation represented in FIG. 3 can be collectedand processed alongside data from the mode of operation represented inFIG. 1 in the manner represented schematically in FIG. 4. For example,the two modes of operation may be operated sequentially, closelysuccessively or simultaneously to provide enhanced characterisationinformation for the composition of imaged objects.

In accordance with the example embodiment three linear array detectorsare illustrated in series. It would be understood that even a pair ofdetectors would generate a pair of images from which monocular movementparallax could be obtained, and that in a practical system it might wellbe desirable to have a larger plurality of detectors in series. Threedetectors are presented by way of illustration only. Nevertheless, aneffective manipulatable and animatable image series can be obtained froma relatively small number of linear detectors in series, especially ifsuitable algorithms are used within the imaging system to generateintermediate images from collected data.

1. An apparatus for generating and displaying an image of an objectcomprising: a radiation source and a series of at least two and no morethan five linear radiation detectors spaced therefrom to define ascanning zone therebetween; means to cause an object to move relative toand through the scanning zone in use; a direct image generationapparatus configured to generate an image from the output of at leastone linear detector; an intermediate image generation apparatusconfigured to generate at least one intermediate image from at least oneadjacent pair of linear detectors, by processing the output of said pairof detectors and generating an image representative of an outputintermediate between the two said detector outputs; the direct andintermediate image generation apparatus adapted such that at least fiveimages are generated in total; an image display adapted to successivelydisplay the images and monocular movement parallax between the images.2. An apparatus in accordance with claim 1 wherein the linear detectorsmaking up the laterally spaced series are arranged such that thedistance between them changes in order to maintain a constant angularseparation between each array.
 3. An apparatus in accordance with claim2 further comprising alignment means to allow for alignment of eachlinear detector.
 4. An apparatus in accordance with claim 1 theapparatus comprises a collimator adapted to allow a user to select twoto five beams from a single radiation source.
 5. An apparatus inaccordance with claim 1 comprising a laterally spaced series of at leastthree but no more than five linear detectors and an image generationapparatus adapted to generate a series of at least six images forsuccessive display.
 6. An apparatus in accordance with claim 1 whereinthe image display is a monocular display adapted to successively displayimages as individual two-dimensional images.
 7. An apparatus inaccordance with claim 6 wherein the image display is a two dimensionalvideo display screen.
 8. An apparatus in accordance with claim 1 whereinthe intermediate image generation apparatus is configured to generatetwo or more intermediate images from at least one adjacent pair ofdetectors which show a gradual transition between the images produced bythe adjacent pair of detectors by processing the output of the pair ofdetectors and generating image representatives of a transition betweenthe two said detected outputs.
 9. An apparatus in accordance with claim1 further comprising control means to enable a user to displaysuccessive images at variable speeds in order to enable a smoothtransition for a human observer between each image and/or in a forwardand reverse order.
 10. An apparatus in accordance with claim 1 whereineach linear detector in the laterally spaced series comprises a lineararray of photodiode cells.
 11. An apparatus in accordance with claim 1wherein at least some of the linear detectors comprise dual energydetectors.
 12. An apparatus in accordance with claim 1 wherein at leastsome of the linear detectors comprise detectors configured to generatespectroscopic information about transmitted radiation, in that thedetector exhibits a spectroscopically variable response across at leasta substantial part of a source spectrum allowing spectroscopicinformation to be retrieved.
 13. An apparatus in accordance with claim12 including image processing means adapted to represent spectroscopicresolution of the source spectrum in the generated image.
 14. Anapparatus in accordance with claim 13 wherein the image processing meansis adapted to process the spectroscopically resolved data into aplurality of bands and apply different colors to represent each suchband in the generated image.
 15. An apparatus in accordance with claim12 wherein the linear detector is fabricated from a material selected toexhibit a direct variable photoelectric response to different parts ofthe source spectrum.
 16. An apparatus in accordance with claim 15wherein the detector comprises a material or materials selected from agroup consisting of cadmium telluride, cadmium zinc telluride (CZT),cadmium manganese telluride (CMT), germanium, lanthanum bromide, andthorium bromide.
 17. An apparatus in accordance with claim 1 wherein themeans to cause an object to move relative to and through the scanningzone comprises an object conveyance means in the form of a planarconveyor to convey an object through a scanning zone defined by a staticsource and detector array.
 18. An apparatus in accordance with claim 1comprising an X-ray source and at least two and no more than five linearradiation detectors spaced therefrom.
 19. An apparatus in accordancewith claim 18 comprising a tungsten X-ray source.
 20. A method ofobtaining an image of an object comprising the steps of: providing aradiation source and a series of at least two and no more than fivelinear radiation detectors spaced therefrom to define a scanning zonetherebetween; causing an object to move relative to and through thescanning zone; generating a direct image from the output of at least onelinear detector; generating at least one intermediate image from atleast one adjacent pair of linear detectors, by processing the output ofsaid pair of detectors and generating an image representative of anoutput intermediate between the two said detector outputs; such that atleast five images are generated in total; displaying such imagessuccessively and thus displaying a monocular movement parallax betweenthe images.
 21. A method in accordance with claim 20 wherein the imagesare displayed successively as individual two-dimensional images.
 22. Amethod in accordance with claim 20 wherein a laterally spaced series ofat least three but no more than five linear detectors is provided togenerate a series of at least six images for successive display.
 23. Amethod in accordance with claim 20 wherein the method comprisesgenerating at least two intermediate images from at least one adjacentpair, and where applicable generating at least one intermediate imagefrom a plurality of linear detectors, by processing the output of the oreach said pair of detectors and generating an image representative of anoutput intermediate between the two said detected outputs.
 24. A methodin accordance with claim 20 wherein the successive images are displayedat custom definable refresh rates and directions under observer controlto facilitate interpretation.
 25. A method in accordance with claim 20wherein images generated by at least some of the linear detectors areresolved into two energy bands.
 26. A method in accordance with claim 20wherein images generated by at least some of the linear detectors areresolved spectroscopically across at least a substantial part of asource spectrum, and spectroscopic resolution of transmitted radiationobtained from the output of each such linear detector is represented inthe displayed image.
 27. A method in accordance with claim 26 wherein abanded mapping is used to represent spectroscopic resolution in thegenerated image in that the source spectrum is divided into a pluralityof bands and different colours are used to represent each such band inthe displayed image.
 28. A method in accordance with claim 20 furthercomprising the steps of: in a first mode of operation, generating aplurality of images from a laterally spaced series of linear arrays ofdetectors in the manner of claim 20; and further in a second mode ofoperation, generating a single collimated primary curtain beam, directedat a particular linear detector within the series of detectors, usingthe other detectors to detect diffracted secondary beams, processingthis information from the other detectors to give additional informationconcerning scattered radiation.
 29. A method in accordance with claim 28wherein both modes of operation are applied sequentially or closelysuccessively or effectively simultaneously.
 30. A method of inaccordance with claim 20 comprising providing an X-ray radiation sourceand a series of at least two and no more than five linear X-raydetectors.